PET is a specialized radiology procedure that generates three-dimensional images of functional processes in a target organ or tissue of a subject. Specifically, in PET studies, biologically active molecules carrying radioactive tracer molecules are first introduced into the subject. The PET system then detects pairs of gamma rays emitted indirectly by the tracer and reconstructs a three-dimensional image of the tracer concentration within the subject by analyzing the detected signals. Because the biologically active molecules used in PET studies are natural substrates of metabolism at the target organ or tissue, PET can evaluate the physiology (functionality) of the target organ or tissue, as well as its biochemical properties. Changes in these properties of the target organ or tissue may provide information for the identification of the onset or progression of a disease before an anatomical change relating to the disease become detectable by other diagnostic tests, such as computed tomography (CT) or magnetic resonance imaging (MRI).
Furthermore, the high sensitivity of PET—in the picomolar range—may allow the detection of small amounts of radio-labeled markers in vivo. PET may be used in conjunction with other diagnostic tests to achieve simultaneous acquisition of both structural and functional information of the subject. Examples include a PET/CT hybrid system, a PET/MR hybrid system.
PET and CT data of a subject may be obtained using a PET/CT hybrid system. The CT data may be applied in the attenuation correction of the PET data. During a scan in the PET/CT system, a subject may undergo respiratory motion. When the scanning is performed for chest or upper abdomen examinations, respiratory motion of the lungs and/or cardiac motion of the heart of the subject may lead to a mismatch between the PET data and the CT data. The mismatch may subsequently cause artifacts in the PET image, which in turn may affect an interpretation of the PET image, or a diagnosis performed on the basis of the PET image. A CT scan is quick so that the subject may be asked to hold his or her breath during the CT scan. A PET scan is relatively slow and the subject needs to breathe during the PET scan, which may lead to a mismatch between the CT data and the PET data. Thus, it is desirable to develop a method and system for matching the CT data acquired when the subject is in a breath-hold status and the PET data acquired when the subject is in a breathing status to reduce the effect of respiratory and/or cardiac motion of the subject and improve the quality of a PET image reconstructed accordingly.